Timing estimation in an OFDM receiver

ABSTRACT

A timing estimation system and methodology are provided. In particular, a first pilot is employed in conjunction with three acquisition stages. In the first stage, an attempt is made to observe the leading edge of the correlation curve associated with the first pilot symbol. In the second stage, a determination is made to confirm a leading edge was detected in the first stage by attempting to observe a trailing edge of the correlation curve. Furthermore, during this second stage, a frequency loop is updated to account for frequency offset. The third stage is for observing the trailing edge of the curve if it was not already observed in stage two. Upon detection of receipt of the first pilot, a second pilot can subsequently be employed to acquire fine symbol timing.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/540,086 entitled “Improved Initial Timing Estimation in an OFDMReceiver,” filed on Jan. 28, 2004. This application is related to U.S.application Ser. No. 11/020,413 entitled “Frame Synchronization andInitial Symbol Timing Acquisition System and Method,” filed on Dec. 22,2004 and is also related to U.S. application Ser. No. 10/931,324entitled “Synchronization in a Broadcast OFDM System using Time DivisionMultiplexed Pilots,” filed on Aug. 3, 2004. The entireties of theaforementioned applications are incorporated herein by reference.

BACKGROUND

I. Field

The following description relates generally to data communication andmore particularly toward signal acquisition and synchronization.

II. Background

There is an increasing demand for high capacity and reliablecommunication systems. Today, data traffic originates primarily frommobile telephones as well as desktop or portable computers. As timepasses and technology evolves, it is foreseeable that there will beincreased demand from other communication devices some of which have notbeen developed as of yet. For example, devices not currently thought ofas communication devices such as appliances as well other consumerdevices, will generate huge amounts of data for transmission.Furthermore, present day devices such as mobile phones and personaldigital assistants (PDAs), among others, will not only be more prevalentbut also demand unprecedented bandwidth to support large and complexinteractive and multimedia applications.

While data traffic can be transmitted by way of wire, demand forwireless communication is currently and will continue to skyrocket. Theincreasing mobility of people of our society requires that technologyassociated therewith be portable as well. Thus, today many peopleutilize mobile phones and PDAs for voice and data transmission (e.g.,mobile web, email, instant messaging . . . ). Additionally, growingnumbers of people are constructing wireless home and office networks andfurther expecting wireless hotspots to enable Internet connectivity inschools, coffee houses, airports and other public places. Still furtheryet, there continues to be a large-scale movement toward integration ofcomputer and communication technology in transportation vehicles such ascars, boats, planes, trains, etc. In essence, as computing andcommunication technologies continue to become more and more ubiquitousdemand will continue to increase in the wireless realm in particular asit is often the most practical and convenient communication medium.

In general, the wireless communication process includes both a senderand a receiver. The sender modulates data on a carrier signal andsubsequently transmits that carrier signal over a transmission medium(e.g., radio frequency). The receiver is then responsible for receivingthe carrier signal over the transmission medium. More particularly, thereceiver is tasked with synchronizing the received signal to determinethe start of a signal, information contained by the signal, and whetheror not the signal contains a message. However, synchronization iscomplicated by noise, interference and other factors. Despite suchobstacles, the receiver must still detect or identify the signal andinterpret the content to enable communication.

At present, there are many conventional spread frequency modulationtechnologies being employed. With these technologies, the power of anarrow band information signal is spread or enlarged across a largetransmission frequency band. This spreading is advantageous at leastbecause such transmissions are generally immune to system noise due tothe small spectral power density. However, one known problem with suchconventional systems is that multipath delay spread begets interferenceamongst a plurality of users.

One of the standards rapidly gaining commercial acceptance is orthogonalfrequency division multiplexing (OFDM). OFDM is a parallel transmissioncommunication scheme where a high-rate data stream is split over a largenumber of lower-rate streams and transmitted simultaneously overmultiple sub-carriers spaced apart at particular frequencies or tones.The precise spacing of frequencies provides orthogonality between tones.Orthogonal frequencies minimize or eliminate crosstalk or interferenceamongst communication signals. In addition to high transmission rates,and resistance to interference, high spectral efficiency can be obtainedas frequencies can overlap without mutual interference.

However, one problem with OFDM systems is that they are especiallysensitive to receiver synchronization errors. This can cause degradationof system performance. In particular, the system can lose orthogonalityamongst subcarriers and thus network users. To preserve orthogonality,the transmitter and the receiver must be synchronized. In sum, receiversynchronization is paramount to successful OFDM communications.

Accordingly, there is a need for a novel system and method ofexpeditious and reliable initial frame synchronization.

SUMMARY

The following presents a simplified summary in order to provide a basicunderstanding of some aspects and embodiments disclosed hereinafter.This summary is not an extensive overview nor is it intended to identifykey/critical elements. Its sole purpose is to present some concepts orprinciples in a simplified form as a prelude to the more detaileddescription that is presented later.

In an aspect, a method of timing estimation comprises receiving a streamof input signals at least some being associated with a pilot symbol,generating correlation outputs forming a correlation curve from thesignals and delayed copies thereof, detecting a potential leading edgeof the correlation curve from correlation outputs, and detecting atrailing edge of the curve from correlation outputs.

In another aspect, a computer implemented method of timing estimationcomprises receiving broadcast signals that transmit at least a pluralityof wireless symbols, detecting a potential leading edge of a correlatoroutput associated with a first pilot symbol, and detecting a trailingedge of the correlator output.

In another aspect, a computer implemented method of timing estimationcomprises receiving a stream of broadcast input signals at least somebeing associated with a pilot symbol, generating correlation outputsthat form a correlation curve over time from the signals and delayedcopies thereof, detecting a leading edge of the correlation curve, anddetecting a trailing edge of the correlation curve.

In another aspect, a timing estimation system comprises a delayedcorrelator component that receives a stream of input samples, correlatesan input samples with delayed versions thereof, and generates aplurality of outputs forming a correlation curve, a leading edgecomponent that receives outputs, compares the outputs with a threshold,and generates a signal if it detects a potential leading edge of thecorrelation curve, and a trailing edge component that upon receipt ofthe signal from the confirmation component compares additional outputsto the threshold to locate the trailing edge of the correlation curve.

In another aspect, a timing estimation system comprises means forreceiving a stream of signals at least a portion of which are associatedwith a pilot symbol, means for generating correlation outputs from thesignals and delayed copies thereof, and means for detecting a leadingedge and a trailing edge from the correlation outputs.

In yet another aspect, a microprocessor that executes instructions forperforming a method of timing estimation comprises generatingcorrelation metrics from signal samples and delayed copies thereof, anddetecting a leading edge and a trailing edge by comparing the metrics toa threshold.

In yet another aspect, a system of timing estimation comprises a firstcomponent that receives a plurality of data packets comprising at leasta pilot symbol, a second component that generates correlation metricsfrom the data packets, a third component that analyzes the metricsovertime to determine whether a pilot symbol has been received, thepilot symbol is received upon detection of metric values consistentlyless than a threshold for a first number of times, followed by a metricvalues greater than or equal to the threshold for a second number oftimes, followed by metric values consistently less than the thresholdfor a third number of times.

To the accomplishment of the foregoing and related ends, certainillustrative aspects and embodiments are described herein in connectionwith the following description and the annexed drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other aspects will become apparent from the followingdetailed description and the appended drawings described in briefhereinafter.

FIG. 1 is a block diagram of coarse frame detection system.

FIG. 2 a is graph of a correlation curve in an ideal single pathenvironment.

FIG. 2 b is a graph of a correlation curve in a real multipathenvironment.

FIG. 3 is a block diagram of an embodiment of a confirmation component.

FIG. 4 is a block diagram of an embodiment of a trailing edge component.

FIG. 5 is a block diagram of an embodiment of a delayed correlatorcomponent.

FIG. 6 is a block diagram of an embodiment of a fine frame detectionsystem.

FIG. 7 is a flow chart diagram of an initial coarse frame detectionmethodology.

FIG. 8 is a flow chart diagram of a leading edge detection methodology.

FIG. 9 is a flow chart diagram of a leading edge confirmation and flatzone detection methodology.

FIG. 10 a is a flow chart diagram of a leading edge confirmation andflat zone detection methodology.

FIG. 10 b is a flow chart diagram of a leading edge confirmation andflat zone detection methodology.

FIG. 11 is a flow chart diagram of a trailing edge detectionmethodology.

FIG. 12 is a flow chart diagram of a frame synchronization methodology.

FIG. 13 is a schematic block diagram of a suitable operating environmentfor various aspects and embodiments.

FIG. 14 is a diagram of an embodiment of a super-frame structure for usein an OFDM system.

FIG. 15 a is diagram of an embodiment of a TDM pilot-1.

FIG. 15 b is diagram of an embodiment of a TDM pilot-2.

FIG. 16 is a block diagram of an embodiment of TX data and pilotprocessor at a base station.

FIG. 17 is a block diagram of an embodiment of OFDM modulator at a basestation.

FIG. 18 a is a diagram of a time-domain representation of TDM pilot-1.

FIG. 18 b is a diagram of a time-domain representation of TDM pilot-2.

FIG. 19 is a block diagram of an embodiment of synchronization andchannel estimation unit at a wireless device.

FIG. 20 is a block diagram of an embodiment of symbol timing detectorthat performs timing synchronization based on the pilot-2 OFDM symbol.

FIG. 21 a is a timing diagram of the processing for a TDM pilot-2 OFDMsymbol.

FIG. 21 b is a timing diagram of an L₂-tap channel impulse response fromIDFT unit.

FIG. 21 c is a plot of the energy of channel taps at different windowstarting positions.

FIG. 22 is a diagram of a pilot transmission scheme with a combinationof TDM and FDM pilots.

FIG. 23 is a flow chart diagram of a detail acquisition procedure inaccordance with an embodiment.

FIG. 24 shows TDM Pilot1 in the frequency domain in accordance with anembodiment.

FIG. 25 shows in accordance with an embodiment, a TDM Pilot1 in the timedomain with periodic waveform, periodicity 128 samples, and 36 periods.

FIG. 26 shows a TDM Pilot2 in the frequency domain in accordance with anembodiment.

FIG. 27 shows in accordance with an embodiment, a TDM Pilot2 in the timedomain with periodic waveform, periodicity 1024 samples, and fourperiods.

DETAILED DESCRIPTION

Various aspects and embodiments are now described with reference to theannexed drawings, wherein like numerals refer to like or correspondingelements throughout. It should be understood, however, that the drawingsand detailed description thereto are not intended to limit embodimentsto the particular forms disclosed. Rather, the intention is to cover allmodifications, equivalents, and alternatives.

As used in this application, the terms “component” and “system” areintended to refer to a computer-related entity, either hardware, acombination of hardware and software, software, or software inexecution. For example, a component may be, but is not limited to being,a process running on a processor, a processor, an object, an executable,a thread of execution, a program, and/or a computer (e.g. desktop,portable, mini, palm . . . ). By way of illustration, both anapplication running on a computer device and the device itself can be acomponent. One or more components may reside within a process and/orthread of execution and a component may be localized on one computerand/or distributed between two or more computers.

Furthermore, aspects may be implemented as a method, apparatus, orarticle of manufacture using standard programming and/or engineeringtechniques to produce software, firmware, hardware, or any combinationthereof to control a computer to implement the disclosed aspects. Theterm “article of manufacture” (or alternatively, “computer programproduct”) as used herein is intended to encompass a computer programaccessible from any computer-readable device, carrier, or media. Forexample, computer readable media can include but is not limited tomagnetic storage devices (e.g., hard disk, floppy disk, magnetic strips. . . ), optical disks (e.g., compact disk (CD), digital versatile disk(DVD) . . . ), smart cards, and flash memory devices (e.g., card,stick). Additionally it should be appreciated that a carrier wave can beemployed to carry computer-readable electronic data such as those usedin transmitting and receiving electronic mail or in accessing a networksuch as the Internet or a local area network (LAN).

In accordance with the corresponding disclosure, various aspects aredescribed in connection with a subscriber station. A subscriber stationcan also be called a system, a subscriber unit, mobile station, mobile,remote station, access point, base station, remote terminal, accessterminal, user terminal, user agent, or user equipment. A subscriberstation may be a cellular telephone, a cordless telephone, a SessionInitiation Protocol (SIP) phone, a wireless local loop (WLL) station, apersonal digital assistant (PDA), a handheld device having wirelessconnection capability, or other processing device connected to awireless modem.

Turning initially to FIG. 1, frame detection system 100 is depicted.More specifically, system 100 is a receiver side sub-system associatedwith synchronization of wireless symbol transmissions (e.g., OFDMsymbols). Synchronization refers generally to the process performed by areceiver to obtain both frame and symbol timing. As will be described inmore detail in the sections that follow, frame detection is based onidentification of pilot or training symbols transmitted at the start ofa frame or super-frame. In one embodiment, the pilot symbols are timedivision multiplexed (TDM) pilots. In particular, a first pilot symbolcan be employed for coarse estimation of a frame at a symbol boundary,inter alia, while a second pilot symbol can be utilized for to improvesuch estimation. System 100 is primarily concerned with detection of thefirst pilot symbol for frame detection, although it can be utilized inconjunction with the detection of other training symbols. System 100includes delayed correlator component 110, leading edge detectioncomponent 120, confirmation component 130, and trailing edge detectioncomponent 130.

The delayed correlator component 110 receives a stream of digital inputsignals from a wireless device receiver (not shown). The delayedcorrelator component 110 processes the input signals and producesdetection metrics or correlation outputs (S_(n)) associated therewith. Adetection metric or correlation output is indicative of the energyassociated with one pilot sequence. The computation mechanisms thatgenerate detection metrics from streams of input signals will bepresented in detail infra. Detection metrics are provided to a leadingedge component 120, a confirmation component 130, and a trailing edgecomponent 140 for further processing.

Turning briefly to FIGS. 2 a and 2 b, two exemplary diagramsillustrating pilot correlation outputs are provided for purposes ofclarity as well as to facilitate appreciation of one of the problemsidentified and overcome. The correlation diagrams depict a correlatoroutput as captured by the magnitude of the detection metric over time.FIG. 2 a depicts correlator output in a channel without noise. Thecorrelator output clearly has a leading edge, a flat portion, andsubsequently a trailing edge. FIG. 2 b illustrates an exemplarycorrelation curve in a channel subject to multipath effects (e.g., noiseis resident upon the channel). One can observe that a pilot is existent,however it is obscured by channel noise and multipath delay.Conventionally, a single threshold is employed to detect a pilot symbol.In particular, the threshold is used to determine the start of a symbolwhen the correlation values are greater than the set or predeterminedthreshold. In the ideal case of FIG. 2 a, the threshold would be setclose to the flat zone value and a symbol would be detected when itcrosses that value. Subsequently, a count would be initiated todetermine the trailing edge. Alternatively, the trailing edge couldsimply be detected when the curve values dip below the threshold.Unfortunately, such conventional methods and techniques are noteffective in a real multipath environment. As can be ascertained fromFIG. 2 b, the leading edge cannot readily be determined from thecorrelation values as multipath effects can cause the values to bespread and noise can further obscure the leading edge. This can resultin a large number of false positive detections. Furthermore, thespreading of the signal is not conducive to counting samples to detect atrailing edge and noise will prohibit detection of a trailing edge whenvalues dip below the threshold. The techniques disclosed herein providea robust system and method of pilot and frame detection that iseffective at least in a real world multipath environment.

Turning back to FIG. 1, leading edge component 120 can be employed todetect a potential leading edge of a correlation curve (e.g., where thecorrelation curve represents an energy distribution over time). Leadingedge component 120 receives a series of detection metric values (S_(n))from the delayed correlator component 120. Upon receipt, the value iscompared to a fixed or programmable threshold (T). In particular, adetermination is made as to whether S_(n)>=T. If it is, then a count orcounter (e.g., run count) is incremented. Alternatively, if S_(n)<T thenthe counter can be set to zero. The counter thereby stores the number ofconsecutive correlation output values that are above the threshold.Leading edge component 120 monitors this counter to ensure that apredetermined or programmed number of samples have been analyzed.According to an embodiment, this can correspond to when the runcount=64. However, it should be appreciated that this value can bemodified to optimize detection in a particular system in a specificenvironment. This technique is advantageous in that it makes it lesslikely that a leading edge will be falsely detected as a result ofinitial noise or spreading, because the samples must consecutively stayabove a threshold for a length of time. Once the condition(s) aresatisfied, the leading edge component can declare detection of apotential leading edge. Subsequently, a signal can be provided toconfirmation component 130 indicating such.

As the name suggests, confirmation component 130 is operable to confirmthat a leading edge was indeed detected by the leading edge component120. Following a leading edge, a lengthy flat period is expected. Hence,if the flat portion is detected then this increases the confidence thatthe leading edge of the pilot symbol was detected by the leading edgecomponent 120. If not, then a new leading edge will need to be detected.Upon receipt of a signal from the leading edge component 120, theconfirmation component 130 can begin to receive and analyze additionaldetection metric values (S_(n)).

Turning to FIG. 3, a block diagram of one exemplary implementation ofthe confirmation component 130 is depicted to facilitate clarity inunderstanding. Confirmation component 130 can include or be associatedwith a processor 310, a threshold value 320, an interval count 330, ahit count 340, a run count 350, and a frequency accumulator 360.Processor 310 is communicatively coupled with threshold 320, intervalcounter 330, hit counter 340, run counter 350, and the frequencyaccumulator 360. Furthermore, processor 310 is operable to receiveand/or retrieve correlation values S_(n) as well as interact (e.g.,receive and transmit signals) with leading edge component 120 (FIG. 1)and trailing edge component 140 (FIG. 1). The threshold value 320 can bethe same threshold as was employed by the leading edge component 120(FIG. 1). Furthermore, it should be noted that while the threshold valueis illustrated as part of the confirmation component 130 as a hard codedvalue, for instance, the threshold value 320 can be received and/orretrieved from outside the component to, among other things, facilitateprogramming of such value. In brief, interval count 330 can be used indetermining when to update a frequency locked loop to determinefrequency offset employing frequency accumulator 360 as well asdetecting the trailing edge. Hit count 340 can be utilized to detect thesymbol flat zone and run count 350 is used to identify a trailing edge.

Prior to initial processing of correlation values, the processor 310 caninitialize each of die counters 330, 340, and 350. as well as thefrequency accumulator 360 to zero, for example. The processor 310 canthen receive or retrieve a correlation output S_(n) and the threshold320 The interval count 330 can then he incremented to note that a newsample has been retrieved. Each time a new correlation sample isretrieved the interval count 330 can be incremented. The processor 310can subsequently compare the correlation value to threshold 320. IfS_(n) is greater than or equal to the threshold, then the hit count canbe incremented. As per the run count, it can be incremented if S_(n) isless then the threshold 320, otherwise it is set to zero. Similar to theleading edge, run count thus can indicate the number of consecutivesamples below threshold. The count values can be analyzed to determinewhether a leading edge has been detected, there was a false positive, orthe leading edge was otherwise missed (eg., got in to late), among otherthings.

In one embodiment, the confirmation component 130 can determine that theleading edge component 120 detected a false leading edge by examiningthe run count and the hit count. Since the confirmation component shouldbe detecting a flat zone of the correlation curve where the values aregreater than or equal to the threshold, if the hit count is sufficientlylow and the run count is greater than a set value or the hit count andthe run count are substantially equal, then it can be determined thatnoise may have caused incorrect detection of a leading edge. Inparticular, it can be noted that the received correlation values are notconsistent with what is expected. According to one embodiment, thedetermination that a false leading edge can be made when the run countis greater than or equal to 128 and the hit count is less than 400.

A determination can be made by the confirmation component 130 that theleading edge was missed or otherwise detected too late for proper timingby again comparing the values of the run count and the hit count. Inparticular, if the hit count and the run count are sufficiently largesuch a determination can be made. In one embodiment, this can be decidedwhen the run count is greater than or equal to 786 and the hit count isgreater than or equal to 400. Of course, and as with all specific valuesprovided herein, the values can be optimized or adjusted for aparticular frame structure and/or environment.

It should be appreciated that the confirmation component 130 can beginto detect the trailing edge of the curve while it is analyzing the flatzone to decide if a proper leading edge was detected. If the trailingedge is detected, the confirmation component can be successfullyterminated. To detect the trailing edge, the interval count and the runcount can be employed. As noted above, the interval count includes thenumber of input samples received and correlated. The length of the flatzone is known to be within a particular count. Hence, if after detectinga potential leading edge and receiving a proper number of flat zonesamples there is some evidence of a trailing edge, then the confirmationcomponent can declare detection of the trailing edge. The evidence of atrailing edge can be provided by the run count, which counts the numberof consecutive times the correlation value is below the threshold. Inone embodiment the confirmation component 130 can declare detection ofthe trailing edge when the interval count is greater than or equal to34*128 (4352) and the run count is greater than zero.

If the confirmation component fails to detect any one of the above threeconditions then it can simply continue to receive correlation values andupdate the counters. If one of the conditions is detected, the processorcan provide one or more additional checks on the counters to increasethe confidence that one of the conditions has actually occurred. Inparticular, the processor 310 can insist upon a minimum number of hitsin the flat zone as that is what it expected to observe after theleading edge detection. For instance, the processor can test whether thehit count is greater than a set value such as 2000. According to oneembodiment of a frame structure disclosed herein, the expected number ofhits in the flat zone should be 34*128, which over 4,000. However, noisewill temper the actual results so the gating value can be set somewhatbelow 4,000. If the additional conditions are met, the confirmationcomponent 130 can provide a signal to the trailing edge componentalternatively the confirmation component can signal the leading edgecomponent to locate a new leading edge.

It should also be appreciated that the confirmation component 130 canalso provide additional functionality such as saving time instances andupdating frequencies. The subject frame detection system 100 of FIG. 1is providing course detection of the frame and symbol boundaries.Accordingly, some fine-tuning will need to be performed at a later timeto get more precise synchronization. Therefore, at least one timereference should be saved for use later by a fine timing system and/ormethod. According to one embodiment, every time the run count is equalto zero, a time instance can be saved as an estimate of the last timefor the correlation curve flat zone or the time just prior to detectingthe trailing edge. Furthermore, proper synchronization necessitateslocking on the appropriate frequency. Hence, the processor 310 canupdate a frequency locked loop utilizing the frequency accumulator 360at particular times such as when the input is periodic. According to oneembodiment, the frequency locked loop can be updated every 128 inputsamples as tracked by the interval counter, for instance.

Returning to FIG. 1, trailing edge component 140 can be employed todetect the trailing edge if not detected by the confirmation component130. In sum, trailing edge component 140 is operable to detect thetrailing edge or simply time out such that another leading edge can bedetected by leading edge component 120.

Turning to FIG. 4 an embodiment of a trailing edge component 140 isillustrated. The trailing edge component 140 can include or beassociated with processor 410, a threshold 420, an interval count 430and a run count 440. Similar to the other detection components, trailingedge component 140 can receive a plurality of correlation values fromthe delayed correlator component 110 and increment appropriate counts tofacilitate detection of a correlation curve trailing edge associatedwith a first pilot symbol (e.g., a TDM pilot symbol). In particular,processor 410 can compare the correlation value with the threshold 420and populate either or both of the interval count 430 and the run count440. It should be noted that although the threshold 420 is illustratedas part of the trailing edge component it could also be received orretrieved from outside the component such as from a central programmaticlocation. It should also be appreciated of course that processor 410can, prior to its first comparison, initialize the interval count 430and a run count 440 to zero. The interval count 430 stores the number ofcorrelation outputs received. Thus, with each received or retrievedcorrelation value, the processor 410 can increment the interval count430. The run count stores the consecutive number of times thecorrelation value or output is less than the threshold 420. If thecorrelation value is less than a threshold then the processor 410 canincrement the run count 440, otherwise run count 440 can be set to zero.The trailing edge component 140 by way of processor 410, for example,can test whether an interval count value or a run count value has beensatisfied utilizing the interval count 430 and or the run count 440. Forinstance, if the run count 440 attains a certain value the trailing edgecomponent can declare detection of a trailing edge. If not, the trailingedge component 140 can continue to receive correlation values and updatethe counts. If, however, the interval count 430 becomes sufficientlylarge this can indicate that the trailing edge will not be detected anda new leading edge needs to be located. In one embodiment, this valuecan be 8*128 (1024). On the other hand, if the run count 440 hits orexceeds a value this can indicate that a trailing edge has beendetected. According to an embodiment, this value can be 32.

Additionally, it should be appreciated that trailing edge component 140can also save time instances for use in acquisition of fine timing.According to an embodiment, the trailing edge component 140 can save thetime instance whenever the run count equals zero thereby providing atime instance just prior to trailing edge detection. According to oneembodiment and the frame structure described infra, the saved timeinstance can correspond to the 256^(th) sample in the next OFDM symbol(TDM pilot-2). A fine frame detection system can subsequently improveupon that value as discussed in later sections.

FIG. 5 illustrates a delayed correlator component 110 in further detailin accordance with one embodiment. The delayed correlator component 110exploits the periodic nature of the pilot-1 OFDM symbol for framedetection. In an embodiment, correlator 110 uses the following detectionmetric to facilitate frame detection:

$\begin{matrix}{{S_{n} = {{\sum\limits_{i = {n - L_{1} + 1}}^{n}{r_{i - L_{1}} \cdot r_{i}^{*}}}}^{2}},} & {{Eq}(1)}\end{matrix}$where S_(n) is the detection metric for sample period n;

-   -   “*” denotes a complex conjugate; and    -   |x|² denotes the squared magnitude of x.        Equation (1) computes a delayed correlation between two input        samples r_(i) and r_(i−L) ₁ in two consecutive pilot-1        sequences, or c_(i)=r_(i−L) ₁ ·r*_(i). This delayed correlation        removes the effect of the communication channel without        requiring a channel gain estimate and further coherently        combines the energy received by way of the communication        channel. Equation (1) then accumulates the correlation results        for all L₁ samples of a pilot-1 sequence to obtain an        accumulated correlation result C_(n), which is a complex value.        Equation (1) then derives the decision metric or correlation        output S_(n) for sample period n as the squared magnitude of        C_(n). The decision metric S_(n) is indicative of the energy of        one received pilot-1 sequence of length L₁, if there is a match        between the two sequences used for the delayed correlation.

Within delayed correlator component 110, a shift register 512 (of lengthL₁) receives, stores, and shifts the input samples {r_(n)} and providesinput samples {r_(n−L) ₁ } that have been delayed by L₁ sample periods.A sample buffer may also be used in place of shift register 512. A unit516 also receives the input samples and provides the complex-conjugatedinput samples {r*_(n)}. For each sample period n, a multiplier 514multiplies the delayed input sample r_(n−L) ₁ from shift register 512with the complex-conjugated input sample r*_(n) from unit 516 andprovides a correlation result c_(n) to a shift register 522 (of lengthL₁) and a summer 524. Lower-case c_(n) denotes the correlation resultfor one input sample, and upper-case C_(n) denotes the accumulatedcorrelation result for L₁ input samples. Shift register 522 receives,stores, and delays the correlation results {c_(n)} from multiplier 514and provides correlation results {c_(n−L) ₁ } that have been delayed byL₁ sample periods. For each sample period n, summer 524 receives andsums the output C_(n−1) of a register 526 with the result c_(n) frommultiplier 514, further subtracts the delayed result c_(n−L) ₁ fromshift register 522, and provides its output C_(n) to register 526.Summer 524 and register 526 form an accumulator that performs thesummation operation in equation (1). Shift register 522 and summer 524are also configured to perform a running or sliding summation of the L₁most recent correlation results c_(n) through c_(n−L) ₁ ₊₁. This isachieved by summing the most recent correlation result c_(n) frommultiplier 514 and subtracting out the correlation result c_(n−L) ₁ fromL₁ sample periods earlier, which is provided by shift register 522. Aunit 532 computes the squared magnitude of the accumulated output C_(n)from summer 524 and provides the detection metric S_(n).

FIG. 6 depicts a fine frame detection system 600. System 600 includes afine timing component 610 and a data decoder component 620. Fine timingcomponent 610 can receive the time instance saved by the coarse framedetection system 100 (FIG. 1). As mentioned above, this time instancecan correspond to the 256^(th) sample of the next OFDM symbol, which canbe TDM pilot-2. This is somewhat arbitrary yet optimized for channelssubject to multipath effects. The fine timing component 610 can thenutilize the TDM pilot-2 symbol to improve upon this coarse timingestimate (T_(c)). There are many mechanisms to facilitate fine timingincluding those known in the art. According to one embodiment herein, afrequency-locked loop or automatic frequency control loop can beswitched from acquisition to tracking mode, which utilizes a differentalgorithm to compute error and a different tracking loop bandwidth. Datadecoder component 620 can attempt to decode one or more data OFDMsymbols. This is an extra step providing for additional confidence thatthe synchronization has been accomplished. If the data does not decode,a new leading edge will have to be detected again by the leading edgecomponent 120 (FIG. 1). Further detail concerning fine timing isprovided infra.

In view of the exemplary systems described supra, methodologies that maybe implemented will be better appreciated with reference to the flowcharts of FIGS. 7–12. While for purposes of simplicity of explanation,the methodologies are shown and described as a series of blocks, it isto be understood and appreciated that the subject methodologies are notlimited by the order of the blocks, as some blocks may occur indifferent orders and/or concurrently with other blocks from what isdepicted and described herein. Moreover, not all illustrated blocks maybe required to implement the provided methodologies.

Additionally, it should be further appreciated that the methodologiesdisclosed hereinafter and throughout this specification are capable ofbeing stored on an article of manufacture to facilitate transporting andtransferring such methodologies to computer devices. The term article ofmanufacture, as used, is intended to encompass a computer programaccessible from any computer-readable device, carrier, or media.

Turning to FIG. 7, a robust method of initial frame detection isillustrated. The method essentially contains three stages. At 710, thefirst stage, an attempt is made to observe a pilot symbol leading edge.The leading edge can be detected by analyzing a plurality of detectionmetrics or correlation output values produced by a delayed correlator.In particular, the detection metrics (S_(n)) or some function thereof(e.g., S_(n) ² . . . ) can be compared with a threshold value. Potentialdetection of the leading edge can then be predicated on the number oftimes the metric is greater than or equal to the threshold. At 720, thedetected leading edge is confirmed by observing additional correlationvalues and comparing them to the threshold. Here, the correlator outputis again compared to the threshold and observations made regarding thenumber of times the correlator output exceeds the threshold. The processcan stay in this stage for greater than or equal to a predeterminedperiod of time (corresponding to the flat zone) or upon detection of aconsistent trailing edge. It should also be noted that frequency offsetcan be obtained here be updating a frequency accumulator periodically.If neither of the confirmation conditions is met, then there was a falsedetection of a leading edge and the procedure can be initialized andstarted again at 710. At 730, an attempt is made to observe the trailingedge if not previously observed. If the correlator output remains belowthe threshold for a number of consecutive samples, for example 32, TDMpilot detection can be declared and initial frequency acquisitionassumed to be complete. If this condition is not met then the processcan be initialized and started again at 710. The initial OFDM symboltime estimate is based on the trailing edge. The time instance when thecorrelator output goes below the threshold for the first time duringobservation of the trailing edge can be views as an index (e.g.,256^(th) sample) into the next OFDM symbol, here, for example, TDMpilot-2.

FIG. 8 is a flow chart diagram depicting a leading edge detectionmethodology 800. At 810, transmitted input samples are received. Adelayed correlation is performed, at 820, on the received input and adelayed version thereof. A correlation output is then provided todecision block 830. At 830, the correlation output is compared with afixed or programmable threshold value. If the correlation value isgreater than or equal to the threshold a run count or counter isincremented at 840. If the correlation value is less then the thresholdvalue then the run count is set to zero, at 850. The run count is thencompared, at 860, with a predetermined value that is optimized fordetection of a leading edge in a multipath environment. In oneembodiment, the value can be 64 input samples. If the run count is equalto the predetermined value the process is terminated. If the run countis not equal to the value then the additional input values are receivedat 810 and the process in repeated.

FIG. 9 is a flow chart diagram of leading edge confirmation methodology900. Methodology 900 represents the second stage in a coarse or initialframe detection methodology, in which a leading edge detection isconfirmed (or rejected) by way of detection of additional expectedresults, namely a flat zone and/or a trailing edge. At 910, one of amyriad of input samples is received. A delayed correlation is performedon the input sample and a delayed version thereof, at 920, to produce acorrelation output. A plurality of correlator outputs are then analyzedwith respect to a programmable threshold to make subsequentdeterminations. At 930, a determination is made as to whether a falseleading edge was detected, which can result from channel noise, amongother things. This determination can be made if not enough correlationoutput values are above a threshold. At 940, a determination is made asto whether a leading edge was detected too late. In other words, theleading edge was not detected until well into the flat zone region ofthe pilot. At 950, a determination is made as to whether a trailing edgeis being observed. If none of these conditions are true based on thecorrelation outputs received thus far, the process continues at 910where more input samples are received. If any one of the conditions istrue, the process can continue at 960, were an additional determinationis made concerning whether a long enough flat zone has been observed toprovide confidence that it was detected. If yes, the procedure can heterminated. If no, the process can proceed with another method, such asmethod 800 (FIG. 8), to detect a new leading edge 970. In oneembodiment, a new pilot symbol will be transmitted one second after theprevious pilot symbol.

FIG. 10 depicts a more detailed method 1000 of detecting the flat zoneand confirming detection of the lending edge in accordance with aparticular embodiment. In this particular process, three counts orcounters are employed: an interval count, a hit count, and a run count.At 1010, counters are all initialized to zero. At 1012, input samplesare received. The interval count is incremented, at 1014, to indicatereceipt of an input sample. It should also be appreciate that althoughnot specifically denoted in the block diagram, a frequency loop can beupdated every 128 samples as tracked by the interval count. At 1016,delayed correlation is performed utilizing the input sample and atime-delayed version thereof to produce a correlation output (S_(n)). Adetermination is then made, at 1018, as to whether S_(n) is greater thanor equal to a threshold (T). If S_(n)>=T, then the hit count isincremented at 1020 and the method can proceed at 1028. If not, then adetermination is made at 1022 as to whetter S_(n)>T. If yes, then therun count is incremented at 1024. If no, then the run count isinitialized to zero and the time is saved 1026. The saved time thereforeprovides the time instance prior to observation of a trailing edge. Itshould he appreciated that decision block 1022 is not strictly necessaryhere but is provided for clarity as well as to highlight further thatthe order of such method processes does not need to be fixed as shown.The method continues to 1028 where the hit count and the run count arescrutinized to determine if a false leading edge was detected. In oneembodiment, this can correspond to the run count being greater than orequal to 128 and the hit count being less than 400. If it is decidedthat a false positive was detected the process proceeds to 1036 where anew leading edge is located. If a false positive was not able to bedetermined then the process continues at decision block 1030. At 1030,the run count and the hit count are analyzed to determine if the leadingedge was detected late. According to one specific embodiment, this cancorrespond to when the run count is greater than or equal to 768 and thehit count is greater than or equal to 400. If this is the case, theprocess can continue at 1034. If the leading edge was not detected late,then the process proceeds to 1032 where the interval count and the runcount are analyzed to determine if a trailing edge is being observed. Inone embodiment this can be where the interval count is greater than orequal to the 4352 (34*128) and the run count is greater than zero. Inother words, the frill length of the flat zone has been detected and adip below threshold has just been observed. If no, then all threeconditions have failed and the process proceeds to 1012 where more inputsamples are received. If yes, a determination is made at 1034 as enoughvalues have been observed above the threshold to enable the methodologyto determine with confidence that the flat zone has been detected. Morespecifically, the hit count is larger than sonic programmable value. Inone embodiment, the value can be 2000. However, this is some-whatarbitrary. Ideally, the process should see 34*128 (4352) samples abovethreshold, but noise can temper the count. Thus, the programmable valuecan set to an optimal level that provides a particular level ofconfidence that the flat zone has been detected. If the hit count isgreater than the provided value, then the process terminates. If not,the process proceeds to 1036 where a new edge needs to be detected.

FIG. 11 illustrates one embodiment of a trailing edge detectionmethodology 1100. Trailing edge methodology can be employed to detectthe trailing edge of correlation curve associated with a pilot symbol,if not previously detected. At 1110, counters including an interval anda run counter are initialized to zero. At 1112, input samples arereceived. The interval count is incremented corresponding to a receivedsample, at 1114. Each input sample is utilized by a delayed correlatorto produce a correlation output S_(n), at 1116. A decision is made at1118 regarding with the correlation output S_(n) is less than aprogrammable threshold (T). If S_(n)<T, then the run count isincremented 1120 and the process proceeds to 1126. If the correlationoutput is not less than the threshold, then the run counter is set tozero at 1122 and the time instance can be saved at 1124. At 1126, adetermination is made as to whether enough correlation outputs have beenobserved consecutively to confidently declare successful identificationthereof. In one embodiment, this corresponds to a run time greater thanor equal to 32. IT the run time is large enough, the process canterminate successfully, if the run time is not large enough, the processproceeds to decision block 1128. At 1128, the interval counter can beemployed to determine whether the detection method 1100 should be timedout. In one embodiment if the interval count is equal to 8*128(1024) thetrailing edge detection method 1100 times out. If the method does nottimeout at 1128, then additional samples can be received and analyzedstarting again at 1112. If the method does time out at 1128, then thenew pilot leading edge will need to be detected 1130 as the method 1100failed to observe a trailing edge.

FIG. 12 illustrates a frame synchronization methodology 1200. At 1210,the process first waits for automatic gain control (AGC) to settle.Automatic gain control adjusts the input signal to provide a consistentsignal strength or level such that the signal, can be properlyprocessed. At 1212, a frequency locked loop (EEL) accumulator isinitialized. At 1214, a potential leading edge is detected. At 1216, theleading edge can be confirmed by detection of a flat zone and/or atrailing edge. If it is determined that a valid leading edge was notdetected at 1218, then the method returns to 1212. It should beappreciated also that it is at this point where the frequency lockedloop can be updated periodically utilizing the frequency accumulator,for example to acquire the initial frequency offset. At 1212, thetrailing edge can be detected if not previously observed. It is herejust prior to the initial dip of the trailing edge that the time can besaved to be used later for fine timing. If the trailing edge is notdetected at 1222 and was not previously detected then the method returnsto 1212. If the trailing edge was detected then the initial coarsedetection has been completed. The procedure continues at 1224 where thefrequency locked loop is switch to tracking mode. Fine timing isacquired utilizing a second TDM pilot symbol and information provided bythe prior coarse estimate 1226. In particular, the time instance saved(T_(c)) can correspond to a particular sample offset within the secondpilot symbol. In accordance with one embodiment, the saved time samplecan correspond to the 256^(th) sample in the second pilot symbol.Specific algorithms can them be utilized to improve upon that timingestimate as described in later sections. Upon termination of fine timingacquisition, one or more data symbols can be retrieved and an attemptmade to decode such symbols can be undertaken at 1228. If, at 1230, thedecoding was successful then the process terminates. However, if theprocess was not successful then the methodology returns to 1212.

The following is a discussion one of a plurality of suitable operatingenvironments to provide context for particular inventive aspectsdescribed supra. Further, in the interest of clarity and understanding adetailed description is provided of one embodiment of time divisionmultiplexed pilots—TDM pilot-1 and TDM pilot-2.

The synchronization techniques described below and throughout may beused for various multi-carrier systems and for the downlink as well asthe uplink. The downlink (or forward link) refers to the communicationlink from the base stations to the wireless devices, and the uplink (orreverse link) refers to the communication link from the wireless devicesto the base stations. For clarity, these techniques are described belowfor the downlink in an OFDM system.

FIG. 13 shows a block diagram of a base station 1310 and a wirelessdevice 1350 in an OFDM system 1300. Base station 1310 is generally afixed station and may also be referred to as a base transceiver system(BTS), an access point, or some other terminology. Wireless device 1350may be fixed or mobile and may also be referred to as a user terminal, amobile station, or some other terminology. Wireless device 1350 may alsobe a portable unit such as a cellular phone, a handheld device, awireless module, a personal digital assistant (PDA), and the like.

At base station 1310, a TX data and pilot processor 1320 receivesdifferent types of data (e.g., traffic/packet data and overhead/controldata) and processes (e.g., encodes, interleaves, and symbol maps) thereceived data to generate data symbols. As used herein, a “data symbol”is a modulation symbol for data, a “pilot symbol” is a modulation symbolfor pilot, and a modulation symbol is a complex value for a point in asignal constellation for a modulation scheme (e.g., M-PSK, M-QAM, and soon). Processor 1320 also processes pilot data to generate pilot symbolsand provides the data and pilot symbols to an OFDM modulator 1330.

OFDM modulator 1330 multiplexes the data and pilot symbols onto theproper subbands and symbol periods and further performs OFDM modulationon the multiplexed symbols to generate OFDM symbols, as described below.A transmitter unit (TMTR) 1332 converts the OFDM symbols into one ormore analog signals and further conditions (e.g., amplifies, filters,and frequency upconverts) the analog signal(s) to generate a modulatedsignal. Base station 1310 then transmits the modulated signal from anantenna 1334 to wireless devices in the system.

At wireless device 1350, the transmitted signal from base station 1310is received by an antenna 1352 and provided to a receiver unit (RCVR)1354. Receiver unit 1354 conditions (e.g., filters, amplifies, andfrequency downconverts) the received signal and digitizes theconditioned signal to obtain a stream of input samples. An OFDMdemodulator 1360 performs OFDM demodulation on the input samples toobtain received data and pilot symbols. OFDM demodulator 1360 alsoperforms detection (e.g., matched filtering) on the received datasymbols with a channel estimate (e.g., a frequency response estimate) toobtain detected data symbols, which are estimates of the data symbolssent by base station 1310. OFDM demodulator 1360 provides the detecteddata symbols to a receive (RX) data processor 1370.

A synchronization/channel estimation unit 1380 receives the inputsamples from receiver unit 1354 and performs synchronization todetermine frame and symbol timing, as described above and below. Unit1380 also derives the channel estimate using received pilot symbols fromOFDM demodulator 1360. Unit 1380 provides the symbol timing and channelestimate to OFDM demodulator 1360 and may provide the frame timing to RXdata processor 1370 and/or a controller 1390. OFDM demodulator 1360 usesthe symbol timing to perform OFDM demodulation and uses the channelestimate to perform detection on the received data symbols.

RX data processor 1370 processes (e.g., symbol demaps, deinterleaves,and decodes) the detected data symbols from OFDM demodulator 1360 andprovides decoded data. RX data processor 1370 and/or controller 1390 mayuse the frame timing to recover different types of data sent by basestation 1310. In general, the processing by OFDM demodulator 1360 and RXdata processor 1370 is complementary to the processing by OFDM modulator1330 and TX data and pilot processor 1320, respectively, at base station1310.

Controllers 1340 and 1390 direct operation at base station 110 andwireless device 1350, respectively. Memory units 1342 and 1392 providestorage for program codes and data used by controllers 1340 and 1390,respectively.

Base station 1310 may send a point-to-point transmission to a singlewireless device, a multi-cast transmission to a group of wirelessdevices, a broadcast transmission to all wireless devices under itscoverage area, or any combination thereof. For example, base station1310 may broadcast pilot and overhead/control data to all wirelessdevices under its coverage area. Base station 1310 may further transmituser-specific data to specific wireless devices, multi-cast data to agroup of wireless devices, and/or broadcast data to all wirelessdevices.

FIG. 14 shows a super-frame structure 1400 that may be used for OFDMsystem 1300. Data and pilot may be transmitted in super-frames, witheach super-frame having a predetermined time duration (e.g., onesecond). A super-frame may also be referred to as a frame, a time slot,or some other terminology. For the embodiment shown in FIG. 14, eachsuper-frame includes a field 1412 for a first TDM pilot (or “TDMpilot-1”), a field 1414 for a second TDM pilot (or “TDM pilot-2”), afield 1416 for overhead/control data, and a field 1418 fortraffic/packet data.

The four fields 1412 through 1418 are time division multiplexed in eachsuper-frame such that only one field is transmitted at any given moment.The four fields are also arranged in the order shown in FIG. 14 tofacilitate synchronization and data recovery. Pilot OFDM symbols infields 1412 and 1414, which are transmitted first in each super-frame,may be used for detection of overhead OFDM symbols in field 1416, whichis transmitted next in the super-frame. Overhead information obtainedfrom field 1416 may then be used for recovery of traffic/packet datasent in field 1418, which is transmitted last in the super-frame.

In an exemplary embodiment, field 1412 carries one OFDM symbol for TDMpilot-1, and field 1414 also carries one OFDM symbol for TDM pilot-2. Ingeneral, each field may be of any duration, and the fields may bearranged in any order. TDM pilot-1 and TDM pilot-2 are broadcastperiodically in each frame to facilitate synchronization by the wirelessdevices. Overhead field 1416 and/or data field 1418 may also containpilot symbols that are frequency division multiplexed with data symbols,as described below.

The OFDM system has an overall system bandwidth of BW MHz, which ispartitioned into N orthogonal subbands using OFDM. The spacing betweenadjacent subbands is BW/N MHz. Of the N total subbands, M subbands maybe used for pilot and data transmission, where M<N. and the remainingN−M subbands may be unused and serve as guard subbands. In anembodiment, the OFDM system uses an OFDM structure with N=4096 totalsubbands, M=4000 usable subbands, and N−M=96 guard subbands. In general,any OFDM structure with any number of total, usable, and guard subbandsmay be used for the OFDM system.

As described supra, TDM pilots 1 and 2 may be designed to facilitatesynchronization by the wireless devices in the system. A wireless devicemay use TDM pilot-1 to detect the start of each frame, obtain a coarseestimate of symbol timing, and estimate frequency error. The wirelessdevice may subsequently use TDM pilot-2 to obtain more accurate symboltiming.

FIG. 15 a shows an embodiment of TDM pilot-1 in the frequency domain.For this embodiment, TDM pilot-1 comprises L₁ pilot symbols that aretransmitted on L₁ subbands, one pilot symbol per subband used for TDMpilot-1. The L₁ subbands are uniformly distributed across the N totalsubbands and are equally spaced apart by S₁ subbands, where S₁=N/L₁. Forexample, N=4096, L₁=128, and S₁=32. However, other values may also beused for N, L₁, and S₁. This structure for TDM pilot-1 can (1) providegood performance for frame detection in various types of channelincluding a severe multi-path channel, (2) provide a sufficientlyaccurate frequency error estimate and coarse symbol timing in a severemulti-path channel, and (3) simplify the processing at the wirelessdevices, as described below.

FIG. 15 b shows an embodiment of TDM pilot-2 in the frequency domain.For this embodiment, TDM pilot-2 comprises L₂ pilot symbols that aretransmitted on L₂ subbands, where L₂>L₁. The L₂ subbands are uniformlydistributed across the N total subbands and are equally spaced apart byS₂ subbands, where S₂=N/L₂. For example, N=4096, L₂=2048, and S₂=2.Again, other values may also be used for N, L₂, and S₂. This structurefor TDM pilot-2 can provide accurate symbol timing in various types ofchannel including a severe multi-path channel. The wireless devices mayalso be able to (1) process TDM pilot-2 in an efficient manner to obtainsymbol timing prior to the arrival of the next OFDM symbol, which is canoccur immediately after TDM pilot-2, and (2) apply the symbol timing tothis next OFDM symbol, as described below.

A smaller value is used for L₁ so that a larger frequency error can becorrected with TDM pilot-1. A larger value is used for L₂ so that thepilot-2 sequence is longer, which allows a wireless device to obtain alonger channel impulse response estimate from the pilot-2 sequence. TheL₁ subbands for TDM pilot-1 are selected such that S₁ identical pilot-1sequences are generated for TDM pilot-1. Similarly, the L₂ subbands forTDM pilot-2 are selected such that S₂ identical pilot-2 sequences aregenerated for TDM pilot-2.

FIG. 16 shows a block diagram of an embodiment of TX data and pilotprocessor 1320 at base station 1310. Within processor 1320, a TX dataprocessor 1610 receives, encodes, interleaves, and symbol mapstraffic/packet data to generate data symbols.

In an embodiment, a pseudo-random number (PN) generator 1620 is used togenerate data for both TDM pilots 1 and 2. PN generator 1620 may beimplemented, for example, with a 15-tap linear feedback shift register(LFSR) that implements a generator polynomial g(x)=x¹⁵+x¹⁴+1. In thiscase, PN generator 1620 includes (1) 15 delay elements 1622 a through1622 o coupled in series and (2) a summer 1624 coupled between delayelements 1622 n and 1622 o. Delay element 1622 o provides pilot data,which is also fed back to the input of delay element 1622 a and to oneinput of summer 1624. PN generator 1620 may be initialized withdifferent initial states for TDM pilots 1 and 2, e.g., to‘011010101001110’ for TDM pilot-1 and to ‘010110100011100’ for TDMpilot-2. In general, any data may be used for TDM pilots 1 and 2. Thepilot data may be selected to reduce the difference between the peakamplitude and the average amplitude of a pilot OFDM symbol (i.e., tominimize the peak-to-average variation in the time-domain waveform forthe TDM pilot). The pilot data for TDM pilot-2 may also be generatedwith the same PN generator used for scrambling data. The wirelessdevices have knowledge of the data used for TDM pilot-2 but do not needto know the data used for TDM pilot-1.

A bit-to-symbol mapping unit 1630 receives the pilot data from PNgenerator 1620 and maps the bits of the pilot data to pilot symbolsbased on a modulation scheme. The same or different modulation schemesmay be used for TUM pilots 1 and 2. In an embodiment, QPSK is used forboth TDM pilots 1 and 2. In this case, mapping unit 1630 groups thepilot data into 2-bit binary values and further maps each 2-bit value toa specific pilot modulation symbol. Each pilot symbol is a complex valuein a signal constellation for QPSK. If QPSK is used for the TDM pilots,then mapping unit 1630 maps 2L₁ pilot data bits for TDM pilot 1 to L₁pilot symbols and further maps 2L₂ pilot data bits for TDM pilot 2 to L₂pilot symbols. A multiplexer (Mux) 1640 receives the data symbols fromTX data processor 1610, the pilot symbols from mapping unit 1630, and aTDM_Ctrl signal from controller 1340. Multiplexer 1640 provides to OFDMmodulator 1330 the pilot symbols for the TDM pilot 1 and 2 fields andthe data symbols for the overhead and data fields of each frame, asshown in FIG. 14.

FIG. 17 shows a block diagram of an embodiment of OFDM modulator 1330 atbase station 1310. A symbol-to-subband mapping unit 1710 receives thedata and pilot symbols from TX data and pilot processor 1320 and mapsthese symbols onto the proper subbands based on a Subband_Mux_Ctrlsignal from controller 1340. In each OFDM symbol period, mapping unit1710 provides one data or pilot symbol on each subband used for data orpilot transmission and a “zero symbol” (which is a signal value of zero)for each unused subband. The pilot symbols designated for subbands thatare not used are replaced with zero symbols. For each OFDM symbolperiod, mapping unit 1710 provides N “transmit symbols” for the N totalsubbands, where each transmit symbol may be a data symbol, a pilotsymbol, or a zero symbol. An inverse discrete Fourier transform (IDFT)unit 1720 receives the N transmit symbols for each OFDM symbol period,transforms the N transmit symbols to the time domain with an N-pointIDFT, and provides a “transformed” symbol that contains N time-domainsamples. Each sample is a complex value to be sent in one sample period.An N-point inverse fast Fourier transform (IFFT) may also be performedin place of an N-point IDFT if N is a power of two, which is typicallythe case. A parallel-to-serial (P/S) converter 1730 serializes the Nsamples for each transformed symbol. A cyclic prefix generator 1740 thenrepeats a portion (or C samples) of each transformed symbol to form anOFDM symbol that contains N+C samples. The cyclic prefix is used tocombat inter-symbol interference (ISI) and intercarrier interference(ICI) caused by a long delay spread in the communication channel. Delayspread is the time difference between the earliest arriving signalinstance and the latest arriving signal instance at a receiver. An OFDMsymbol period (or simply, a “symbol period”) is the duration of one OFDMsymbol and is equal to N+C sample periods.

FIG. 18 a shows a time-domain representation of TDM pilot-1. An OFDMsymbol for TDM pilot-1 (or “pilot-1 OFDM symbol”) is composed of atransformed symbol of length N and a cyclic prefix of length C. Becausethe L₁ pilot symbols for TDM pilot 1 are sent on L₁ subbands that areevenly spaced apart by S₁ subbands, and because zero symbols are sent onthe remaining subbands, the transformed symbol for TDM pilot 1 containsS₁ identical pilot-1 sequences, with each pilot-1 sequence containing L₁time-domain samples. Each pilot-1 sequence may also be generated byperforming an L₁-point IDFT on the L₁ pilot symbols for TDM pilot 1. Thecyclic prefix for TDM pilot-1 is composed of the C rightmost samples ofthe transformed symbol and is inserted in front of the transformedsymbol. The pilot-1 OFDM symbol thus contains a total of S₁+C/L₁ pilot-1sequences. For example, if N=4096, L₁=128, S₁=32, and C=512, then thepilot-1 OFDM symbol would contain 36 pilot-1 sequences, with eachpilot-1 sequence containing 128 time-domain samples.

FIG. 18 b shows a time-domain representation of TDM pilot-2. An OFDMsymbol for TDM pilot-2 (or “pilot-2 OFDM symbol”) is also composed of atransformed symbol of length N and a cyclic prefix of length C. Thetransformed symbol for TDM pilot 2 contains S₂ identical pilot-2sequences, with each pilot-2 sequence containing L₂ time-domain samples.The cyclic prefix for TDM pilot 2 is composed of the C rightmost samplesof the transformed symbol and is inserted in front of the transformedsymbol. For example, if N=4096, L₂=2048, S₂=2, and C=512, then thepilot-2 OFDM symbol would contain two complete pilot-2 sequences, witheach pilot-2 sequence containing 2048 time-domain samples. The cyclicprefix for TDM pilot 2 would contain only a portion of the pilot-2sequence.

FIG. 19 shows a block diagram of an embodiment of synchronization andchannel estimation unit 1380 at wireless device 1350 (FIG. 13). Withinunit 1380, a frame detector 100 (as described in detail in supra)receives the input samples from receiver unit 1354, processes the inputsamples to detect for the start of each frame, and provides the frametiming. A symbol timing detector 1920 receives the input samples and theframe timing, processes the input samples to detect for the start of thereceived OFDM symbols, and provides the symbol timing. A frequency errorestimator 1912 estimates the frequency error in the received OFDMsymbols. A channel estimator 1930 receives an output from symbol timingdetector 1920 and derives the channel estimate.

As described in further detail in FIG. 1, frame detector 100, performsframe synchronization by detecting, for example, for TDM pilot-1 in theinput samples from receiver unit 1354. For simplicity, the instantdetailed description assumes that the communication channel is anadditive white Gaussian noise (AWGN) channel. The input sample for eachsample period may be expressed as:r _(n) =x _(n) +w _(n),  Eq (2)where n is an index for sample period;

-   -   x_(n) is a time-domain sample sent by the base station in sample        period n;    -   r_(n) is an input sample obtained by the wireless device in        sample period n; and    -   w_(n) is the noise for sample period n.

Frequency error estimator 1912 estimates the frequency error in thereceived pilot-1 OFDM symbol. This frequency error may be due to varioussources such as, for example, a difference in the frequencies of theoscillators at the base station and wireless device, Doppler shift, andso on. Frequency error estimator 1912 may generate a frequency errorestimate for each pilot-1 sequence (except for the last pilot-1sequence), as follows:

$\begin{matrix}{{{\Delta\; f_{l}} = {\frac{1}{G_{D}}{{Arg}\left\lbrack {\sum\limits_{i = 1}^{L_{1}}{r_{l,i} \cdot r_{l,{i + L_{1}}}^{*}}} \right\rbrack}}},} & {{Eq}(1)}\end{matrix}$where r_(l,i) is the i-th input sample for the l-th pilot-1 sequence;

-   -   Arg (x) is the arc-tangent of the ratio of the imaginary        component of x over the real component of x, or Arg (x)=arctan        [Im(x)/Re(x)];    -   G_(D) is a detector gain, which is

${G_{D} = \frac{2{\pi \cdot L_{1}}}{f_{samp}}};{and}$

-   -   Δf_(l) is the frequency error estimate for the l-th pilot-1        sequence.        The range of detectable frequency errors may be given as:

$\begin{matrix}{{{2{\pi \cdot L_{1} \cdot \frac{{\Delta\; f_{l}}}{f_{samp}}}} < {\pi/2}},{{{or}\mspace{14mu}{{\Delta\; f_{l}}}} < \frac{f_{samp}}{4 \cdot L_{1}}},} & {{Eq}(2)}\end{matrix}$where f_(samp) is the input sample rate. Equation (2) indicates that therange of detected frequency errors is dependent on, and inverselyrelated to, the length of the pilot-1 sequence. Frequency errorestimator 1912 may also be implemented within the frame detectorcomponent 100 and more specifically by way of the delayed correlatorcomponent 110 since the accumulated correlation results are alsoavailable from summer 524.

The frequency error estimates may be used in various manners. Forexample, the frequency error estimate for each pilot-1 sequence may beused to update a frequency tracking loop that attempts to correct forany detected frequency error at the wireless device. Thefrequency-tracking loop may be a phase-locked loop (PLL) that can adjustthe frequency of a carrier signal used for frequency downconversion atthe wireless device. The frequency error estimates may also be averagedto obtain a single frequency error estimate Δf for the pilot-1 OFDMsymbol. This Δf may then be used for frequency error correction eitherprior to or after the N-point DFT within OFDM demodulator 160. Forpost-DFT frequency error correction, which may be used to correct afrequency offset Δf that is an integer multiple of the subband spacing,the received symbols from the N-point DFT may be translated by Δfsubbands, and a frequency-corrected symbol {tilde over (R)}_(k) for eachapplicable subband k may be obtained as {tilde over (R)}_(k)={tilde over(R)}_(k+Δf). For pre-DFT frequency error correction, the input samplesmay be phase rotated by the frequency error estimate Δf, and the N-pointDFT may then be performed on the phase-rotated samples.

Frame detection and frequency error estimation may also be performed inother manners based on the pilot-1 OFDM symbol. For example, framedetection may be achieved by performing a direct correlation between theinput samples for pilot-1 OFDM symbol with the actual pilot-1 sequencegenerated at the base station. The direct correlation provides a highcorrelation result for each strong signal instance (or multipath). Sincemore than one multipath or peak may be obtained for a given basestation, a wireless device would perform post-processing on the detectedpeaks to obtain timing information. Frame detection may also be achievedwith a combination of delayed correlation and direct correlation.

FIG. 20 shows a block diagram of an embodiment of symbol timing detector1920, which performs timing synchronization based on the pilot-2 OFDMsymbol. Within symbol timing detector 1920, a sample buffer 2012receives the input samples from receiver unit 1354 and stores a “sample”window of L₂ input samples for the pilot-2 OFDM symbol. The start of thesample window is determined by a unit 2010 based on the frame timingfrom frame detector 100.

FIG. 21 a shows a timing diagram of the processing for the pilot-2 OFDMsymbol. Frame detector 100 provides the coarse symbol timing (denoted asT_(C)) based on the pilot-1 OFDM symbol. The pilot-2 OFDM symbolcontains S₂ identical pilot-2 sequences of length L₂ (e.g., two pilot-2sequences of length 2048 if N=4096 and L₂=2048). A window of L₂ inputsamples is collected by sample buffer 912 for the pilot-2 OFDM symbolstarting at sample period T_(W). The start of the sample window isdelayed by an initial offset OS_(init) from the coarse symbol timing, orT_(W)=T_(C)+OS_(init). The initial offset does not need to be accurateand is selected to ensure that one complete pilot-2 sequence iscollected in sample buffer 2012. The initial offset may also be selectedsuch that the processing for the pilot-2 OFDM symbol can be completedbefore the arrival of the next OFDM symbol, so that the symbol timingobtained from the pilot-2 OFDM symbol may be applied to this next OFDMsymbol.

Referring back to FIG. 20, a DFT unit 2014 performs an L₂-point DET onthe L₂ input samples collected by sample buffer 2012 and provides L₂frequency-domain values for L₂ received pilot symbols. If the start ofthe sample window is not aligned with the start of the pilot-2 OFDMsymbol (i.e., T_(W)≠T_(S)), then the channel impulse response iscircularly shifted, which means that a front portion of the channelimpulse response wraps around to the back. A pilot demodulation unit2016 removes the modulation on the L₂ received pilot symbols bymultiplying the received pilot symbol R_(k) for each pilot subband kwith the complex-conjugate of the known pilot symbol P*_(k) for thatsubband, or R_(k)·P*_(k)*. Unit 2016 also sets the received pilotsymbols for the unused subbands to zero symbols. An IDFT unit 2018 thenperforms an L₂-point IDET on the L₂ pilot demodulated symbols andprovides L₂ time-domain values, which are L₂ taps of an impulse responseof the communication channel between base station 1310 and wirelessdevice 1350.

FIG. 21 b shows the L₂-tap channel impulse response from IDFT unit 2018.Each of the L₂ taps is associated with a complex channel gain at thattap delay. The channel impulse response may be cyclically shifted, whichmeans that the tail portion of the channel impulse response may wraparound and appear in the early portion of the output from IDFT unit2018.

Referring back to FIG. 20, a symbol timing searcher 2020 may determinethe symbol timing by searching for the peak in the energy of the channelimpulse response. The peak detection may be achieved by sliding a“detection” window across the channel impulse response, as indicated inFIG. 21 b. The detection window size may be determined as describedbelow. At each window starting position, the energy of all taps fallingwithin the detection window is computed.

FIG. 21 c shows a plot of the energy of the channel taps at differentwindow starting positions. The detection window is shifted to the rightcircularly so that when the right edge of the detection window reachesthe last tap at index L₂, the window wraps around to the first tap atindex 1. Energy is thus collected for the same number of channel tapsfor each window starting position.

The detection window size L_(W) may be selected based on the expecteddelay spread of the system. The delay spread at a wireless device is thetime difference between the earliest and latest arriving signalcomponents at the wireless device. The delay spread of the system is thelargest delay spread among all wireless devices in the system. If thedetection window size is equal to or larger than the delay spread of thesystem, then the detection window, when properly aligned, would captureall of the energy of the channel impulse response. The detection windowsize L_(W) may also be selected to be no more than half of L₂ (orL_(W)≦L₂/2) to avoid ambiguity in the detection of the beginning of thechannel impulse response. The beginning of the channel impulse responsemay be detected by (1) determining the peak energy among all of the L₂window starting positions and (2) identifying the rightmost windowstarting position with the peak energy, if multiple window startingpositions have the same peak energy. The energies for different windowstarting positions may also be averaged or filtered to obtain a moreaccurate estimate of the beginning of the channel impulse response in anoisy channel. In any case, the beginning of the channel impulseresponse is denoted as T_(B), and the offset between the start of thesample window and the beginning of the channel impulse response isT_(OS)=T_(B)−T_(W). Fine symbol timing may be uniquely computed once thebeginning of the channel impulse response T_(B) is determined.

Referring to FIG. 21 a, the fine symbol timing is indicative of thestart of the received OFDM symbol. The fine symbol timing T_(S) may beused to accurately and properly place a “DFT” window for eachsubsequently received OFDM symbol. The DFT window indicates the specificN input samples (from among N+C input samples) to collect for eachreceived OFDM symbol. The N input samples within the DFT window are thentransformed with an N-point DFT to obtain N received data/pilot symbolsfor the received OFDM symbol. Accurate placement of the DFT window foreach received OFDM symbol is needed in order to avoid (1) inter-symbolinterference (ISI) from a preceding or next OFDM symbol, (2) degradationin channel estimation (e.g., improper DFT window placement may result inan erroneous channel estimate), (3) errors in processes that rely on thecyclic prefix (e.g., frequency tracking loop, automatic gain control(AGC), and so on), and (4) other deleterious effects.

The pilot-2 OFDM symbol may also be used to obtain a more accuratefrequency error estimate. For example, the frequency error may beestimated using the pilot-2 sequences and based on equation (3). In thiscase, the summation is performed over L₂ samples (instead of L₁ samples)for the pilot-2 sequence.

The channel impulse response from IDFT unit 2018 may also be used toderive a frequency response estimate for the communication channelbetween base station 1310 and wireless device 1350. A unit 2022 receivesthe L₂-tap channel impulse response, circularly shifts the channelimpulse response so that the beginning of the channel impulse responseis at index 1, inserts an appropriate number of zeros after thecircularly-shifted channel impulse response, and provides an N-tapchannel impulse response. A DFT unit 2024 then performs an N-point DFTon the N-tap channel impulse response and provides the frequencyresponse estimate, which is composed of N complex channel gains for theN total subbands. OFDM demodulator 1360 may use the frequency responseestimate for detection of received data symbols in subsequent OFDMsymbols. The channel estimate may also be derived in some other manner.

FIG. 22 shows a pilot transmission scheme with a combination of TDM andFDM pilots. Base station 1310 may transmit TDM pilots 1 and 2 in eachsuper-frame to facilitate initial acquisition by the wireless devices.The overhead for the TDM pilots is two OFDM symbols, which may be smallcompared to the size of the super-frame. The base station 1310 may alsotransmit an FDM pilot in all, most, or some of the remaining OFDMsymbols in each super-frame. For the embodiment shown in FIG. 22, theFDM pilot is sent on alternating sets of subbands such that pilotsymbols are sent on one set of subbands in even-numbered symbol periodsand on another set of subbands in odd-numbered symbol periods. Each setcontains a sufficient number of (L_(fdm)) subbands to support channelestimation and possibly frequency and time tracking by the wirelessdevices. The subbands in each set may be uniformly distributed acrossthe N total subbands and evenly spaced apart by S_(fdm)=N/L_(fdm)subbands. Furthermore, the subbands in one set may be staggered oroffset with respect to the subbands in the other set, so that thesubbands in the two sets are interlaced with one another. As an example,N=4096, L_(fdm)=512, S_(fdm)=8, and the subbands in the two sets may bestaggered by four subbands. In general, any number of subband sets maybe used for the FDM pilot, and each set may contain any number ofsubbands and any one of the N total subbands.

A wireless device may use TDM pilots 1 and 2 for initialsynchronization, for example for frame synchronization, frequency offsetestimation, and fine symbol timing acquisition (for proper placement ofthe DFT window for subsequent OFDM symbols). The wireless device mayperform initial synchronization, for example, when accessing a basestation for the first time, when receiving or requesting data for thefirst time or after a long period of inactivity, when first powered on,and so on.

The wireless device may perform delayed correlation of the pilot-1sequences to detect for the presence of a pilot-1 OFDM symbol and thusthe start of a super-frame, as described above. Thereafter, the wirelessdevice may use the pilot-1 sequences to estimate the frequency error inthe pilot-1 OFDM symbol and to correct for this frequency error prior toreceiving the pilot-2 OFDM symbol. The pilot-1 OFDM symbol allows forestimation of a larger frequency error and for more reliable placementof the DFT window for the next (pilot-2) OFDM symbol than conventionalmethods that use the cyclic prefix structure of the data OFDM symbols.The pilot-1 OFDM symbol can thus provide improved performance for aterrestrial radio channel with a large multi-path delay spread.

The wireless device may use the pilot-2 OFDM symbol to obtain finesymbol timing to more accurately place the DFT window for subsequentreceived OFDM symbols. The wireless device may also use the pilot-2 OFDMsymbol for channel estimation and frequency error estimation. Thepilot-2 OFDM symbol allows for fast and accurate determination of thefine symbol timing and proper placement of the DFT window.

The wireless device may use the FDM pilot for channel estimation andtime tracking and possibly for frequency tracking. The wireless devicemay obtain an initial channel estimate based on the pilot-2 OFDM symbol,as described above. The wireless device may use the FDM pilot to obtaina more accurate channel estimate, particularly if the FDM pilot istransmitted across the super-frame, as shown in FIG. 11. The wirelessdevice may also use the FDM pilot to update the frequency-tracking loopthat can correct for frequency error in the received OFDM symbols. Thewireless device may further use the FDM pilot to update a time trackingloop that can account for timing drift in the input samples (e.g., dueto changes in the channel impulse response of the communicationchannel).

FIG. 23 is a flow chart diagram of a detail initial acquisitionprocedure in accordance with an embodiment. Frequency and OFDM symboltiming are also presented.

The initial acquisition procedure based on a first TDM pilot symbolcomprises three stages. In the first stage, the leading edge of thecorrelation curve is detected. In an embodiment, the leading edge can beconfirmed by detection of a flat zone and/or a trailing edge. Inalternative embodiment, the leading edge is not confirmed, but isassumed.

At 2302, the process waits for the AGC to settle. The AGC adjusts theinput signal to provide a consistent signal strength or level such thatthe signal can be properly processed. At 2304, a frequency locked loop(FLL) frequency accumulator is initialized and a run count isinitialized to zero. The run count counts the number of consecutiveinput samples.

At 2306, the magnitude square of the correlator output S is comparedwith a programmable threshold T. In detail at 2306, for each new inputsample, the process performs the delayed correlation and if(|S_(n)|²>=T), then the run count is incremented, else the run count isre-initialized to zero. S_(n) denotes the correlator output for a samplen.

At 2308, if the correlator output exceeds the threshold for aconsecutive 64 input samples in accordance with an embodiment, then thealgorithm enters into the second stage of the acquisition process.Otherwise, the flow of control proceeds to 2306. Thus, if it isdetermined that a valid leading edge was not detected at 2308, then theprocess returns to 2306. In an alternative embodiment, step 2308 is notincluded since the leading edge is assumed detected and is notconfirmed.

In the second stage, at 2310, an interval count, a hit count, and therun count are initialized to zero. In the second stage, a hit count isincremented each time the correlator output exceeds the threshold. Thealgorithm returns to initial state after it detects that the leadingedge observed was a false one. The algorithm stays in the second stagefor greater than or equal to predetermined period of time or until itobserves a consistent trailing edge of the correlation curve. If thecorrelator output remains below threshold for a consecutive 768 inputsamples, then the algorithm leaves stage two. The closed loop initialfrequency acquisition occurs while the algorithm is in stage two. FLL isupdated once every 128 input samples while in stage two.

At 2312, for each new input sample, the delayed correlation isperformed, the interval count is incremented. If (|S_(n)|²>=T), then thehit count is incremented. If (|S_(n)|²<T), the run count is incremented,else the run count is reinitialized to zero. When the run count equalszero, the time instance is saved, which is used as a buffer pointer. TheFLL is updated when the interval count less than or equal to 32*128 andthe interval count modulo 128 equals zero.

At 2314, a check is made to determine whether (run count is>=128 and thehit count<400) or (run count>=768 and hit count>=400) or (intervalcount>=34*128 and run count>0). If yes, then the flow of controlproceeds to 2316. Otherwise, the flow of control proceeds to 2312.

At 2316, a check determines whether hit count is greater or equal to2000. If yes, then the acquisition process proceeds to 2318. It shouldbe appreciated also that it is at this point where the frequency lockedloop can he updated periodically utilizing the frequency accumulator,for example to acquire the initial frequency offset. If no, then theflow of control proceeds to 2304.

At 2318, the interval count is reinitialized to zero. For each new inputsample, the delayed correlation is performed and the interval count isincremented. If (|S_(n)|²<T), then the run count is incremented, elsethe run count is reinitialized to zero. When the run count equals zero,the time instance is saved.

At 2320, a check is made to determine whether the interval count =8*128or the run count>=32. If no, then the flow of control proceeds to 2318.If yes, then the flow of control proceeds to step 2322. At 2322, a checkis made to determine whether the run count>=32. If no, then the flow ofcontrol proceeds to 2304. If yes, then the flow of control proceeds to2324.

At 2324, detection is declared and the saved instance is the 256^(th)sample in the next OFDM symbol. At 2326, the FLL switches to trackingmode. Fine Timing is acquired using Pilot2. At 2328, OverheadInformation Symbol (OIS) and N data symbols are decoded.

At 2330, a check is made to determine whether OIS/Data decoding wassuccessful. If no, then the flow of control proceeds to 2304. If yes,then acquisition is complete.

At 2320, the trailing edge can be detected if not previously observed.It is here just prior to the initial dip of the trailing edge that thetime can be saved to be used later for fine timing. If the trailing edgeis not detected at 2322 and was not previously detected then the methodreturns to 2304. If the trailing edge was detected then the initialcoarse detection has been completed. The procedure continues at 2326where the frequency locked loop is switched to tracking mode. Finetiming is acquired utilizing a second TDM pilot symbol and informationprovided by the prior coarse estimate. In particular, the time instancesaved (T_(c)) can correspond to a particular sample offset within thesecond pilot symbol. In accordance with one embodiment, the saved timesample can correspond to the 256^(th) sample in the second pilot symbol.Specific algorithms can then be utilized to improve upon that timingestimate as described in later sections. Upon termination of fine timingacquisition, one or more data symbols can be retrieved and an attemptmade to decode such symbols can be undertaken at 2328. If, at 2330, thedecoding was successful then the process terminates. However, if theprocess was not successful then the methodology returns to 2304.

Stage three is for observing the trailing edge if it was not alreadyobserved in stage two. In stage three, if the correlator output remainsbelow threshold for a minimum of consecutive 32 input samples and hitcount during stage two exceeded another programmable threshold, then TDMpilot1 detection is declared and initial frequency acquisition isassumed to be completed. The initial OFDM symbol time estimate is basedon the trailing edge. The time instance when the correlator output goesbelow the threshold for the first time during the observation of thetrailing edge, is taken as the 256^(th) sample of next OFDM symbol (TDMpilot 2). If the hit count is found to be less than the programmablethreshold or a consistent trailing edge during a time-out period of 1024input sample in stage three is not observed, then the algorithm resetsthe counts and the frequency accumulator of the FLL and returns to thefirst stage looking for another leading edge.

Upon successful detection of the TDM pilot symbol 1, pilot symbol 2 isused to acquire the fine OFDM symbol timing. Thereafter, an attempt ismade to decode the OIS and subsequent N data OFDM symbols. The AFC loopoperates in the tracking mode after the first TDM pilot. If the decodingof OIS and data OFDM symbol fails, then it is assumed that AFC loopfailed to converge and entire acquisition process is repeated during thenext frame.

In a detailed embodiment, a frame structure includes two TDM pilotsymbols to acquire the initial time, frequency and framesynchronization. The TDM pilot symbols are known OFDM symbols designedfor initial acquisition. The pilot symbols are placed at the beginningof each superfame preceding an OIS signal field.

The first TDM pilot symbol has 125 non-zero sub-carriers in thefrequency domain. These non-zero sub-carriers are uniformly spaced. Eachpair of consecutive non-zero sub-carriers is separated by 31 zerosub-carriers. The frequency index of 16 is assigned to the firstnon-zero sub-carrier. A PN sequence of length 125 is used for the binarymodulation of non-zero sub-carriers. Such a structure in frequencydomain results in a periodic sequence in time domain with periodicityequal to 128 samples. Thus, the first pilot symbol in time domain has 36replicas (including the cyclic prefix) of a sequence of length 128. Sucha pilot structure not only simplifies the implementation, it is alsowell suited for frequency estimation and frame boundary detection insevere multi-path channel.

Though the first pilot can provide sufficiently accurate frequencyestimate in severe multi-path channel, it cannot provide fine OFDMsymbol timing in such a channel. It can only provide coarse OFDM symboltiming along with the frame boundary. The second pilot symbol has beenincluded to obtain the fine OFDM symbol timing. Its structure is chosenwith the goal that it share the same hardware resources meant for symboltime tracking. Recall that symbol time tracking uses 1000 FDM pilotsub-carriers staggered over two adjacent OFDM symbols. The second TDMpilot symbol has 1000 non-zero sub-carriers in the frequency domain atthe locations corresponding to the staggered FDM pilot locations in twoadjacent data OFDM symbols. The pilot sub-carriers are uniformly spacedand each pair of non-zero sub-carriers is separated by three zerosub-carriers. The frequency index of 48 is assigned to the first pilotsub-carrier. A PN sequence of length 1000 is used for the binarymodulation of the pilot sub-carriers. Such a structure in frequencydomain results in a periodic sequence in time domain with periodicityequal to 1024 samples. Thus the second pilot symbol in time domain has 4replicas (excluding the cyclic prefix) of a sequence of length 1024.This structure reuses the same hardware used for symbol time trackingand achieves the fine symbol timing well within the second pilot symbol.

The initial frequency offset needs to be acquired with a factor of 2uncertainty in VCXO gain sensitivity. In addition, 68 micro sec of PDMtime constant is assumed.

It would be apparent to those skilled in the art that other values mightbe used rather than those shown in the detailed embodiment immediatelyabove.

In accordance with an embodiment, the frame structure with TDM pilotsand structure of pilot symbols in frequency and time domains are shownFIGS. 24 to 27.

FIG. 24 shows TDM Pilot1 in the frequency domain in accordance with anembodiment. Every 32 sub-carrier is non-zero. 4096 sub-carriers 2402 areshown. FIG. 25 shows in accordance with an embodiment, a TDM Pilot1 inthe time domain with periodic waveform, periodicity 128 samples, and 36periods. 128 samples 2502 are shown. FIG. 26 shows a TDM Pilot2 in thefrequency domain in accordance with an embodiment. Every fourthsub-carrier is non-zero. 4096 sub-carriers 2602 are shown. FIG. 27 showsin accordance with an embodiment, a TDM Pilot2 in the time domain withperiodic waveform, periodicity 1024 samples, and four periods. 512samples 2702 and 1024 samples 2704 are shown.

The synchronization techniques described herein may be implemented byvarious means. For example, these techniques may be implemented inhardware, software, or a combination thereof. For a hardwareimplementation, the processing units at a base station used to supportsynchronization (e.g., TX data and pilot processor 120) may beimplemented within one or more application specific integrated circuits(ASICs), digital signal processors (DSPs), digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), processors, controllers, micro-controllers,microprocessors, other electronic units designed to perform thefunctions described herein, or a combination thereof. The processingunits at a wireless device used to perform synchronization (e.g.,synchronization and channel estimation unit 180) may also be implementedwithin one or more ASICs, DSPs, and so on.

For a software implementation, the synchronization techniques may beimplemented in combination with program modules (e.g., routines,programs, components, procedures, functions, data structures, schemas .. . ) that perform the various functions described herein. The softwarecodes may be stored in a memory unit (e.g., memory unit 1392 in FIG. 13)and executed by a processor (e.g., controller 190). The memory unit maybe implemented within the processor or external to the processor.Moreover, those skilled in the art will appreciate that the subjectinventive methods may be practiced with other computer systemconfigurations, including single-processor or multiprocessor computersystems, mini-computing devices, mainframe computers, as well aspersonal computers, hand-held computing devices, microprocessor-based orprogrammable consumer electronics, and the like.

As used herein, OFDM may also include an orthogonal frequency divisionmultiple access (OFDMA) architecture where multiple users share the OFDMchannels.

What has been described above includes examples of various aspects andembodiments. It is, of course, not possible to describe everyconceivable combination of components or methodologies. Variousmodifications to these embodiments will be readily apparent to those ofskill in the art, and the generic principles defined herein may beapplied to other embodiments without departing from the spirit or scopeof the aforementioned embodiments. Thus, the disclosed embodiments arenot intended to be limited to the aspects and embodiments shown anddescribed herein but is to be accorded the widest scope consistent withthe principles and novel features and techniques disclosed herein.Furthermore, to the extent that the term “includes” is used in eitherthe detailed description or the claims, such term is intended to beinclusive in a manner similar to the term “comprising” as “comprising”is interpreted when employed as a transitional word in a claim.

1. A method of timing estimation, comprising; receiving a stream ofinput signals at least some being associated with a first pilot symbol;generating correlation outputs forming a correlation curve from thesignals and delayed copies thereof; detecting a potential leading edgeof the correlation curve from correlation outputs; updating a frequencylocked loop (FLL) frequency accumulator responsive to detecting thepotential leading edge of the correlation curve; detecting a trailingedge of the curve from correlation outputs; saving timing informationbased on the time of detecting the trailing edge; and performing finetiming responsive to detecting the trailing edge, the fine timingpreformed based on a second pilot symbol and the saved timinginformation.
 2. The method of claim 1, wherein the input symbols areOFDM symbols.
 3. The method of claim 1, wherein the input symbols areTDM symbols.
 4. The method of claim 1, further comprising decoding anOverhead Information Symbol (OIS) and N data symbols.
 5. A timingestimation system comprising: means for receiving a stream of inputsignals at least some being associated with a first pilot symbol; meansfor generating correlation outputs forming a correlation curve from thesignals and delayed copies thereof; means for detecting a potentialleading edge of the correlation curve from correlation outputs; meansfor updating a frequency locked loop (FLL) frequency accumulatorresponsive to detecting the potential leading edge of the correlationcurve; means for detecting a trailing edge of the curve from correlationoutputs; means for saving timing information based on the time ofdetecting the trailing edge; and means for performing fine timingresponsive to detecting the trailing edge, the fine timing performedbased on a second pilot symbol and the saved timing information.